The present invention relates to a shield for use in plating a substrate to achieve a layer of substantially uniform thickness across the surface of the substrate. Particularly, the present invention relates to a shield for use in plating a substrate used in making master recordings for high density recorded discs, such as video discs.
Recorded discs generally include a spiral groove in the surface of the disc with the recorded information being within the groove. High density record discs, such as a video disc, have a very high packing density of the grooves, about 10,000 grooves per inch. To achieve such high packing density the groove must be very narrow, about 2.7 microns, and very shallow, about 4,000 Angstroms. Because of these fine dimensions, many of the requirements of materials and processes for the replication of these high packing density discs are different from those used previously to make audio discs.
In the replication system used in the audio record technology, record masters are mechanically cut into a lacquer coated substrate and electroformed to produce nickel molding stampers. It has been found to be unsatisfactory to cut the high packing density, fine dimensioned grooves required for video information in a lacquer coating. However, it has been found that a suitable master can be electromechanically cut in a 0.5 mm thick bright copper layer electroplated upon a smoothly machined 0.5 inch (12.5 mm) thick aluminum disc substrate. However, to achieve a groove of uniform depth and width, the electroplated copper layer should be of uniform thickness across the entire surface of the substrate.
In electroplating a substrate by immersing it in a plating solution, the normal thickness distribution of the plated layer is a thick edge and a thinner center. To overcome this non-uniform distribution, the substrate has been partially immersed in the plating solution, with about 35% of its area exposed above the solution level, and rotated about its center during electroplating. Although this improves the thickness uniformity of the plated layer, it still leaves a non-uniform coating with a thicker outer edge and a hump in the layer near the solution level. Therefore, it would be desirable to be able to electroplate a copper layer on a substrate with the layer being of substantially uniform thickness across the entire surface of the substrate.